Method for forming chromium anisotropic metal particles

ABSTRACT

A method for forming dendritic metal powders, comprising the steps of: (1) heating a powder comprising non-dendritic particles, under conditions suitable for initial stage sintering, to form a lightly sintered material; and (2) breaking the lightly sintered material to form a powder comprising dendritic particles. In one embodiment, the lightly sintered material is broken by brushing the material through a screen. 
     Another aspect of the present invention comprises the dendritic particles that are produced by the method described above. These particles can comprise any suitable metal, such as transition metals, rare earth metals, main group metals or metalloids or an alloy of two or more such metals. The particles can also comprise a ceramic material, such as a metal oxide. These particles are characterized by a dendritic, highly anisotropic, morphology arising from the fusion of substantially non-dendritic particles, and by a low apparent density relative to the substantially non-dendritic starting material. The present dendritic particles can be of high purity, and substantially free of carbon contamination.

RELATED APPLICATIONS

This application is a Divisional of U.S. Ser. No. 08/820,762, filed onMar. 19, 1997, which is a Divisional of 08/604,811, filed on Feb. 21,1996, now U.S. Pat. No. 5,814,272 the entire teachings of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

Metal powders are common starting materials for the fabrication ofmetallic structures. Such structures are typically made by packing themetal powder into a mold, then sintering the shaped powder to form acontinuous structure with the desired mechanical properties. Theproperties of the final structure depend strongly upon the morphology ofthe starting powder particles. The particle morphology, for example,determines the packing efficiency of the particles, and, hence, thedensity and porosity of the final structure.

Dendritic or filamentary powders of nickel and iron are commerciallyavailable, for example, INCO® Filamentary Nickel Powder, Type 287(International Nickel Company, Inc., Saddle Brook, N.J.). There are,however, no commercially available dendritic powders of metals otherthan iron, nickel and copper. Powders of most metals can be formed byatomization, which typically yields substantially non-dendritic powderparticles. Electrodeposition is used to prepare powders of iron, copperand silver. These powders can be dendritic, but are expensive to produceand incorporate impurities derived from the anion present in thestarting material (Taubenblat in Powder Metallurgy, Volume 7 of MetalsHandbook, Ninth Edition, American Society of Metals, Metals Park, Ohio).Powders of metallic nickel and iron can also be formed by thermaldecomposition of the highly toxic organometallic compounds nickeltetracarbonyl and iron pentacarbonyl, respectively. Depending upon thedetails of this process, the resulting powders have morphologies whichare either substantially spherical or filamentary.

Dendritic particles of many metals and alloys, however, cannot be formedby metal carbonyl decomposition. Unlike nickel tetracarbonyl and ironpentacarbonyl, other binary metal carbonyl compounds do not thermallydecompose to form elemental metal and carbon monoxide. Moreover, forcertain metals, such as the main group metals, platinum, palladium andthe rare earth metals (lanthanides and actinides), binary carbonylcompounds are unknown (Cotton et al., Advanced Inorganic Chemistry,Wiley: New York, 1021-1051 (1987)). In addition, formation of a metalalloy powder via decomposition of a molecular precursor requires thatthe precursor contain the desired metals in the desired proportions, inorder to achieve the intimate mixing, on the atomic scale, required of asolid solution, such as an alloy. Certain bimetallic carbonyl compoundsare known, but they are generally difficult to produce in macroscopicquantities and none are known to form alloys upon decomposition (Cottonet al. (1987), supra). Furthermore, the method by which the filamentarynickel and iron powders are prepared is not applicable to othersubstantially pure metals and alloys. This method also yields productswith a substantial carbon impurity, particularly in the case of iron.

There is a need for metal membrane filter elements, for a variety ofapplications, fabricated of a variety of metal powders, includingdendritic or filamentary powders, and with increased purity. This isparticularly true when nickel and iron are incompatible with a potentialapplication of the device. For example, such filters could be employedto purify gases used in semiconductor manufacturing. In thisapplication, however, nickel would be disadvantageous, as it catalyzesthe decomposition of certain hydridic reagents frequently used insemiconductor synthesis, such as phosphine, arsine and diborane.

Thus, the need exists for dendritic powders of metals andmetal-containing materials beyond those currently available. Thelimitations of previously known methods for the production of dendriticmetal powders indicate that this need can be met via the development ofnew methods for the formation of such powders.

SUMMARY OF THE INVENTION

The present invention relates to a method for forming dendritic metalparticles, comprising the steps of: (1) heating a powder comprisingnon-dendritic particles, under conditions suitable for the initial stageof sintering, to form a lightly sintered material; and (2) breaking thelightly sintered material to form a powder comprising dendriticparticles. In one embodiment of the method, the powder comprisingnon-dendritic particles is spread or placed in a layer on a suitablesubstrate prior to heating. In another embodiment, the lightly sinteredmaterial is broken by brushing the material through a screen. In anotherembodiment, steps (1) and (2) above are repeated, in sequence, one ormore times.

Another embodiment of the present invention includes the dendriticparticles that can be formed by way of the method described above. Theseparticles can comprise any suitable metal, such as a transition metal, arare earth metal, a main group metal or metalloid or an alloy of two ormore such metals. The particles can also comprise a ceramic material,such as a metal oxide. The particles produced by this method arecharacterized by a dendritic, highly anisotropic, morphology arisingfrom the fusion of substantially non-dendritic particles, and by a lowapparent density relative to the substantially non-dendritic startingmaterial. The present dendritic particles can be of high purity, andsubstantially free of carbon contamination. An additional advantage ofthe present method is that it provides dendritic particles of metals,such as nickel and iron, without the use of highly toxic metal carbonylprecursors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the size distribution of INCONEL® 625 powderparticles as received from the manufacturer and after six initial stagesintering/breaking cycles.

FIG. 2 is a graph illustrating the change in air-laid density ofINCONELO® 625 powder as a function of the number of initial stagesintering/breaking cycles.

FIG. 3A is an energy dispersive x-ray fluorescence spectrum of untreatedINCONEL® 625 powder.

FIG. 3B is an energy dispersive x-ray fluorescence spectrum of INCONELO®625 powder after six initial stage sintering/breaking cycles.

FIG. 4 is a graph showing changes in the density of 316L stainless steelpowder after two successive initial stage sintering/breaking cycles.

FIG. 5 is a graph comparing particle size distributions for untreated316L stainless steel powder and 316L stainless steel powder treated withfour initial stage sintering/breaking cycles.

FIG. 6 is a scanning electron micrograph of the dendritic metalparticles produced as described in Example 1.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for forming dendritic metalparticles. The method comprises the steps of: (1) heating a powdercomprising non-dendritic particles, under conditions suitable forinitial stage sintering, to form a lightly sintered, and, optionally,continuous material; and (2) breaking the lightly sintered, and,optionally, continuous material to form a powder comprising dendriticmetal particles.

Another embodiment of the present invention includes the dendriticparticles that are formed via the method of the present invention. Theseparticles can comprise any suitable metal, including a transition metal,such as scandium, titanium, vanadium, chromium, manganese, iron cobalt,nickel, copper or zinc, or a heavier congener of these metals; a rareearth metal, such as uranium, gadolinium, europium, samarium, ytterbiumor another metal from the lanthanide and actinide series; a main groupmetal, such as lithium, beryllium and or a congener, aluminum, tin,lead, gallium, antimony, or indium; or a metalloid, such as boron,silicon, tellurium, germanium or arsenic. These particles can alsocomprise a monophasic or multiphasic solid solution of one or more ofthese metals in another, such as an alloy. In addition, the particlesformed by the present method can comprise a ceramic material, such as ametal or metalloid oxide or a metal or metalloid nitride.

For the purposes of the present application, the term “dendritic” isintended to mean a highly anisotropic, irregular morphology comprisingone or more filaments individually having one dimension substantiallygreater than the other two. The filaments can be straight or bent andcan also be branched or unbranched. Dendritic particles arecharacterized by low packing efficiencies compared to particles of moreregular morphology and, therefore, form powders of lower density thanthose formed by particles of more regular morphology. The dendriticparticles of the present invention are formed by the fusion of thestarting, substantially nondendritic, particles. Under magnification,the particles can appear as aggregates of the starting particles with asubstantially dendritic morphology. Dendritic powders also formself-supporting green forms and sintered articles of lower density, and,thus, higher porosity, than powders of more regular morphology.

The term “substantially non-dendritic powder” refers to a powdercomprising particles which typically have a non-dendritic morphology.Thus, these particles have at least two dimensions which aresubstantially equivalent, for example, with lengths of the same order ofmagnitude.

The term “lightly sintered material” is intended to mean a materialcreated by the fusion of metal powder particles through the initialstage of sintering, as defined by Randall (Randall in “Powder MetallurgyScience”, second edition, German, ed., Metal Powder Federation Industry(1994), the contents of which are incorporated herein by reference). Inthe initial stage of sintering, or short-range diffusional sintering,bonds form between particles at the particle contacts, resulting in thefusion of metal powder particles with their immediate neighbors only.Thus, the initial stage of sintering yields a brittle structure of lowmechanical strength. For a given material, sintering proceeds slowlybeyond this initial stage at temperatures at the lower end of thematerial's sintering range. For the purposes of the present inventionthe term “initial stage sintering” refers to the sintering of a powderunder conditions in which sintering does not proceed substantiallybeyond the initial stage.

The term “air-laid density” as used herein is the measured density of apowder after it is sifted through a screen, and allowed to fall throughthe air into a mold or container of known volume. This method ofmeasuring density is highly reproducible, such as for the dendriticpowders of the type described herein.

The term “metal” as used herein refers to any metallic or metalloidchemical element or an alloy of two or more of these elements. Preferredmetals include members of the transition metals, such as platinum,chromium, nickel, and alloys, such as stainless steel and INCONEL® 625.

The term “ceramic” as used herein refers to any combination of one ormore metallic or metalloid elements with one or more non-metallic maingroup elements, forming a non-molecular solid material, such as a metalor metalloid oxide or nitride. Examples include various silicates,tungsten trioxide, tantalum nitride, and silicon nitride.

The present method allows the production of dendritic metal powders thathave heretofore been inaccessible, and with a purity limited only by thepurity of available non-dendritic starting materials. Metal-containingmaterials formed by decomposition of an organometallic precursorgenerally incorporate carbon impurities. For example, the filamentarynickel powders marketed by International Nickel Company, Inc., such asINCO® Filamentary Nickel Powder, Type 287, have a stated typical purityof 99.6%, with a maximum specified carbon content of 0.25%. In contrast,powders formed by atomization can have higher purities. For example, anon-filamentary nickel powder marketed by Aldrich Chemical Company(Milwaukee, Wis.) has a stated purity of 99.999%. As the present methodis carried out under relatively mild conditions of temperature andpressure, and can be carried out in an inert or reducing atmosphere, thechemical composition of the powder does not substantially change as aresult of this process. Thus, the product dendritic powders havesubstantially the purity of the starting, substantially non-dendritic,powders. Use of the Aldrich nickel powder as the starting material inthe present method, therefore, is expected to yield a dendritic nickelpowder of substantially equivalent purity, a marked increase in purityover presently available filamentary or dendritic nickel powders.

The present method can, thus, provide dendritic particles substantiallyfree of carbon contamination because it does not rely uponcarbon-containing starting materials. The carbon content of thedendritic particles thus produced can be substantially less than 0.20%,depending upon the available non-dendritic powders of the material ofinterest.

The conditions, including temperature, at which the initial stage ofsintering takes place depend upon the material of interest and can bereadily determined by the person of ordinary skill in the art. Theinitial stage of sintering for a given material, generally andoptimally, takes place at the lower end of the material's sinteringtemperature range; sintering moves beyond the initial stage only slowlyunder these conditions. The heating is preferably performed under vacuum(for example, at a pressure on the order of 10⁻⁶ torr), in an inertatmosphere such as helium, argon or dinitrogen, or in a reducingatmosphere, such as dihydrogen. In the two latter cases, the pressure ispreferably between about 0-5 atmospheres or slightly higher, and, morepreferably between 0 atmospheres and about 1.5 atmospheres. Theseconditions are preferred to avoid exposure of the metal particles tooxygen, which at elevated temperatures will react with many metals toform a metal oxide surface. A reducing atmosphere, such as dihydrogen,is capable of purifying the particles by removing contaminants such asoxygen, nitrogen, carbon and sulfur. Of course, where the desiredmaterial is a metal oxide, the atmosphere can comprise oxygen.

The duration of step (1), above, can be sufficient to effect initialstage sintering throughout the starting powder sample. The length oftime necessary will depend upon the material of interest, the amount ofpowder being treated, the thickness of the powder sample, and the sizeof the metal particles.

In one embodiment, the powder comprising non-dendritic particles isspread or placed on a plate, or other suitable substrate, prior toheating, preferably in a uniform layer of thickness about twocentimeters or less. This increases the uniformity of initial stagesintering throughout the sample.

In a further embodiment of the present invention, a sample of powder canbe cycled through steps (1) and (2) of the method two or more times insuccession. As used herein, the terms “cycle” and “initial stagesintering/breaking cycle” refer to the sequential completion of steps(1) and (2) of the method described above. As described in Examples 1and 2, the density of a sample of powder decreases with each cycle ofthis process. Thus, with the nickel/chromium/iron/molybdenum alloypowder INCONEL® 625 as the starting material, the air-laid density ofthe powder as purchased was 3.7 g/cm³. After three initial stagesintering/breaking cycles, the air-laid density was 3.0 g/cm³, and,after six cycles, the air-laid density was further reduced to 2.38g/cm³. In yet another embodiment, the lightly-sintered material of step(2) is broken by agitation, for example, by stirring. This step can beautomated and carried out within the furnace using means known in theart.

The reduction in air-laid density following each of several successiveinitial stage sintering/breaking cycles is interpreted as indicatingincreased dendritic character of the powder particles with each cycle.Thus, the longer the dendritic powder particles and the greater thedegree of branching, the lower the packing efficiency, and, hence, thedensity of the powder, is expected to be. This effect has beendemonstrated for the INCO® (International Nickel Co.) filamentary nickelpowders, where the lower density powders are composed of particles whichare, on average, longer, thinner and more highly branched, while thehigher density powders are composed of particles which are shorter andthicker. Thus, the present method enables the preparation of powders ofdifferent densities, with the density of a given powder dependent uponthe number of initial stage sintering/breaking cycles used to preparethe powder. The method of the present invention, can, therefore, reducethe air-laid density of a metal powder starting material by at leastabout 20%, preferably about 30% and, more preferably, about 40%.

Materials suitable for this method include sinterable materialsavailable as substantially non-dendritic powders. These includesubstantially pure metals from the alkali and alkaline earth families,the transition metals, main group metals such as aluminum, tin and lead,the rare earth metals (lanthanides and actinides) and metalloids, suchas silicon, germanium and arsenic. Alloys can also be employed, as canceramic materials, such as metal or metalloid oxides and metal ormetalloid nitrides. The starting non-dendritic particles can be of anysize, but, in a preferred embodiment have diameters on the order of 10μm or less.

In a preferred embodiment, the lightly-sintered material is mechanicallybroken by brushing the material through a screen. The mesh size of thisscreen sets an upper limit for particle diameter. The powder comprisingdendritic particles obtained in this way can be further sifted to removeparticles smaller than a pre-determined size. In this way, a powdersample with a well-defined particle size range can be prepared.

Another aspect of the present invention comprises dendritic particlesthat are prepared by the method of the present invention. These includedendritic particles substantially comprising a single metallic ormetalloid element, such as any member of the alkali metals, the alkalineearth metals, the transition metals, the main group metals or metalloidsor the rare earth metals (i.e., the lanthanide or actinide metals). Alsoincluded are dendritic particles prepared by the method of the presentinvention comprising an alloy of two or more metals or metalloids fromthe foregoing groups of elements. In addition, the present inventionencompasses dendritic particles prepared by the present methodcomprising a ceramic material, such as a metal or metalloid oxide, ametal or metalloid nitride, or a mixed metal, for example, ternary,oxide.

An additional embodiment of the present invention includes dendriticparticles substantially comprising metallic or metalloid elements whichare members of the alkali or alkaline earth metals, Groups 3-7, Group 9or Groups 12-16, or the rare earth metals. Also included are dendriticparticles comprising platinum, palladium, ruthenium, osmium, silver andgold. The present invention also encompasses dendritic particlescomprising an alloy comprising two or more metallic elements. Inaddition, the present invention further includes dendritic particlescomprising a ceramic material, such as a metal or metalloid oxide, ametal or metalloid nitride, or a mixed metal oxide.

The dendritic metal powders of the present invention have severaladvantages. As starting materials in the fabrication of the metalmembrane filters described in U.S. Pat. No. 5,487,771, the contents ofwhich are incorporated by reference in their entirety, they provideaccess to such high porosity filters compatible with a variety ofchemical substances and conditions of use. For example, a sample ofdendritic INCONEL® 625 powder, prepared as described in Example 1, wasused as the starting material for the fabrication of such a metalmembrane filter element. The green form intermediate had a density of3.13 g/cm³ (63% porous), while the final sintered article had a densityof 3.44 g/cm³ (60% porous). It is noteworthy that the treated powderyields a sintered article which is of lower density than the untreatedINCONEL® 625 powder (3.7 g/cm³). Thus, the treated powder provides asintered article of lower density (and higher porosity) than can beattained with the untreated powder in the absence of a binding agent.

Due to their greater surface area relative to particles of more regularmorphology, dendritic powders of metals and metal oxides that are usefulas heterogeneous catalysts are expected to display enhanced activityrelative to powders of the same materials with more regularmorphologies. Also, dendritic powders are highly compressible and canform a self-supporting green form, the structure resulting fromcompression of a powder in a mold, prior to sintering. Such a green formcan, thus, be sintered without its mold. This is advantageous becausesintering a powder in a mold can lead to deformation of the mold, andcontamination of the metal powder.

Treatment of a metal powder by one or more initial stagesintering/breaking cycles also yields a powder with improved flowabilitycompared to the starting material. This is due to the increase inparticle size resulting from the initial stage sintering/breaking cycle,which offsets the decrease in flowability generally observed whenparticle morphology is made more irregular. Thus, the powders of thepresent invention will be of use in applications in which powderflowability is important.

The invention will now be further and specifically described by thefollowing examples.

EXAMPLES Example 1 Preparation of a Dendritic Ni/Cr/Mo/Fe Alloy Powder

MATERIAL

INCONEL® 625 Ni/Cr/Mo/Fe alloy powder (−3 μm cut) was obtained fromInternational Nickel Company, Inc. This powder, as received, had anair-laid density of 3.70 g/cm³, while the density of non-powder solidINCONEL® 625 is 8.44 g/cm³.

METHOD

A 200 g portion of INCONEL® 625 powder was placed on a molybdenum plateand lightly pressed with a similar top plate to form a uniform layer ofapproximately 2.0 cm thickness. The plate holding the powder was placedunder vacuum (10⁻⁶ torr) in a vacuum furnace. The temperature of thefurnace was then increased at the rate of 25° C. per minute until thetemperature reached 760° C. This temperature was maintained for 30minutes, then the furnace was allowed to cool to room temperature,thereby obtaining a lightly sintered material. The lightly sinteredpowder was then placed on a screen with a sieve size of 100 μm andgently broken up into a powder by brushing it through the screen. Theresulting powder was again lightly sintered as described above and theresulting material was broken into powder. A total of six initial stagesintering/breaking cycles were performed on this powder sample.

A sample of the powder prepared by the method outlined above was used toprepare a metal membrane filter element according to the proceduredescribed in U.S. Pat. No. 5,487,771.

RESULTS

The air-laid density of this powder after six treatments was 2.38 g/cm³.As shown in FIG. 1, the treatment results in a much broader sizedistribution, shifted toward larger sizes, than was present in thestarting material. The distribution ranges from less than 5 μm togreater than 30 μm with the greatest proportion falling between 5 and 20μm. FIG. 2 shows how the air-laid density of the powder changes withadditional treatment cycles. There is a monotonic decrease in air-laiddensity with increasing number of treatments. This indicates that thepowder particle morphology becomes more irregular with each treatment.

FIG. 3A and FIG. 3B show EDS spectra of INCONEL® 625 powder before andafter six initial stage sintering/breaking cycles. No significantdifferences in elemental composition of the treated and untreatedpowders are noted.

A metal membrane filter element was fabricated from the treated INCONEL®625 powder via the method disclosed in U.S. Pat. No. 5,487,771. Asindicated above, the treated powder had an air-laid density of 2.38g/cm³. The green form produced by compressing this powder in a mold, hada density of 3.13 g/cm³ (63% porous), and the final filter element,following sintering, had a density of 3.44 g/cm³ (60% porous).

Example 2 Preparation of a Dendritic Stainless Steel Powder

MATERIAL

316L stainless steel powder (−10 μm cut) was obtained from Ametek(Ametek Specialty Metal Products Division, Eighty-four, PA). The powder,as received, had an air-laid density of 2.79 g/cm³.

METHOD

The method described in Example 1 was followed with two exceptions.First, a 100 g sample of the 316L stainless steel starting material wasused. Second, the temperature was ramped to a maximum temperature of800° C., which was maintained for 30 minutes. The initial stagesintering/mechanical breaking procedure was performed a total of fourtimes. A metal membrane filter element was fabricated from the resultingdendritic 316L stainless steel powder following the procedure disclosedin U.S. Pat. No. 5,487,771.

RESULTS

FIG. 4 illustrates the changes in air-laid density of the 316L stainlesssteel powder accompanying a series of initial stage sintering/breakingcycles. A monotonic decrease in density with an increasing number ofcycles is observed. Following four treatment cycles the air-laid densitywas 1.54 g/cm³.

FIG. 5 illustrates the change in particle size distribution followingfour initial stage sintering/breaking cycles. The starting powder has arelatively narrow size distribution, with the majority of particleswithin the 3 μm to 15 μm range. After four initial stagesintering/breaking cycles, however, the distribution is much broader,and shifted toward larger sizes, with the majority of particles now ofsize greater than 20 μm.

The metal membrane filter element produced from the dendritic 316Lstainless steel powder was 3.13 g/cm³ (61% porous), while the density ofthe compacted green form was 2.83 g/cm³ (65% porous).

EQUIVALENTS

Those skilled in the art will recognize or be able to ascertain using nomore than routine experimentation many equivalents to the specificembodiments of the invention described herein. Such equivalents areintended to be encompassed in the scope of the following claims.

What is claimed is:
 1. A chromium powder, wherein said chromium powderis produced by a method comprising the steps of: (a) heating a startingchromium powder comprising non-dendritic particles under conditionssuitable for initial stage sintering, thereby forming a lightly sinteredmaterial; and (b) breaking the lightly sintered material, therebyforming a chromium powder comprising anisotropic metal particles havingirregular morphology, said particles comprising aggregated and fusednon-dendritic metal particles and having an air-laid density which islower than the air-laid density of said starting chromium powder.
 2. Thechromium powder of claim 1 wherein the starting chromium powdercomprises an alloy of chromium.
 3. The chromium powder of claim 2wherein the starting chromium powder consists essentially of chromium.4. A metal powder comprising anisotropic metal particles havingirregular morphology, said particles comprising aggregated and fusednon-dendritic metal particles, wherein the metal is an alloy ofchromium.
 5. A metal powder comprising anisotropic metal particleshaving irregular morphology, said particles comprising aggregated andfused non-dendritic metal particles, wherein the metal consistsessentially of chromium.